But the optimal level of fats in the diet, and which kinds, remain a topic of heated debate, as does the process by which the government's official dietary guidelines have been developed. In particular, the role of the U.S. Department of Agriculture has been questioned, given its mission to promote food producers' interests.

Do you agree that it's still a good idea to avoid butter and other animal fats?

Christopher D. Gardner, PhD, Stanford University: I would be more comfortable with agreeing to the statement that, "I agree it is a good idea to avoid excessive amounts of butter and animal fats, just as it is in general a good idea to avoid excess of just about anything in our daily diet." When we focus on single foods or specific nutrients to avoid, it reinforces the idea that the mere absence of these will be virtuous and healthful.

Connie Diekman, MEd, RD, LD, FADA, Washington University in St. Louis: The goal for limiting animal fats is more of a moderation rather than a "must avoid." Just as important as limiting saturated fats is what do you replace these fats with? Boosting carbohydrates in place of fats is not a good step but choosing polyunsaturated fats instead of animal fats will help lower LDL-C.

Alice H. Lichtenstein, DSc, Tufts University: We should no longer be talking about saturated fat in isolation. The important factor is the relative amount of unsaturated to saturated fat, in favor of the former and limited in the latter. What should be avoided is the replacement of animal and dairy fat with refined carbohydrate, as we saw in the 1990's during the low-fat craze.

Dean Ornish, MD, Preventive Medicine Research Institute: I do think it's wise to avoid butter and animal fats. TIME magazine reported that "butter is not linked to a higher rate of heart disease" but neglected to mention that it IS directly linked with all-cause mortality, which is even more important. Telling people what they want to hear is a good way to sell magazines and books, but it isn't true.

Randomized trials are probably out of the question, so how can we ever determine for sure whether particular foods contribute to long-term adverse outcomes?

David L. Katz, MD, MPH, True Health Initiative: Randomized trials are not out of the question. They are part of it. Importantly though, they are only part of it. I think people sometimes use the idea of science as an excuse for abandoning sense. The only real problem with common sense is that it isn't nearly common enough. Science without sense is very prone to generating answers to all the wrong questions.

Diekman: Until we learn how to use the genetic map to identify what each one of us needs to stay healthy, diet advice will rely on RCT's that look at risk factors – which are not the same as RCT's that focus on disease outcomes – and then extrapolate those outcomes to diet guidelines. Diet guidelines provide just that, guidelines that offer information on how to structure eating plans that allow for variance but still will promote health.

Lichtenstein: It is unlikely any single food determines long-term health outcomes. Emphasis should be focused on the whole diet. The important point, sometimes missed, is balance -- more of some things and less of others.

Gardner: As far as I know, we still don't have any randomized trials that parachutes can be used effectively to save the lives of people jumping out of planes, and yet skydivers continue to use them!? Some questions don't lend themselves to randomized trials, and that is particularly true of food and nutrition questions that involve how to prevent long-term chronic degenerative non-communicable diseases. Fortunately we have mechanistic studies, observational studies, and randomized trials of short-term outcomes involving risk factors for chronic diseases.

Lee M. Kaplan, MD, PhD, Massachusetts General Hospital: Even without randomized trials, there are good epidemiological studies demonstrating the adverse effects of different macro- and micronutrients. However, evidence is now accumulating that different people respond differently to different nutrients. We need more research on the medical effects of dietary manipulation. These studies need to be carefully controlled, and they need to be large enough to account for the wide variability among different people, but they don't necessarily need to be randomized, controlled trials.

Should the USDA be involved in developing dietary guidelines?

Kaplan: The USDA has resources that allow it to do a better job than most other groups in supporting the careful evaluation of nutritional research. Far better than excluding the USDA from the development of national dietary standards would be better transparency and conflict of interest rules that insulates USDA staff and investigators from outside influences.

Frank B. Hu, MD, MPH, PhD, Harvard T.H. Chan School of Public Health: To develop guidelines that can help people make healthy food and beverage choices, it is critical for the process to be evidence-based, transparent, and free of political and industry influences. Unfortunately this has not been the case. For example, the recommendation on reducing red meat intake and environmental sustainability by the 2015 Dietary Guidelines Advisory Committee (DGAC) was removed in the official 2015-2020 Dietary Guidelines for Americans due to pressure from the Congress and the meat industry.

Gardner: Given the obvious and inherent conflict of interest involved, it would be appropriate to lessen the role of the USDA in the development of the national dietary guidelines to that of an advisory role, rather than their current role in overseeing and releasing those updates every 5 years.

Robert Lustig, MD, University of California San Francisco: The USDA in charge of the Dietary Guidelines is like the fox in charge of the hen house. While the DGAC is made up of scientists who want to see science used for good, the USDA has for decades ignored the DGAC's advice, and promulgated that which benefitted the industry at the public's expense. One DGAC chairperson confided in me that the DGAC is ineffective because "it has no teeth."

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